Techniques exist nowadays for juxtaposing virtual pieces of equipment on a single piece of equipment. For this purpose, the operation of a virtual piece of equipment is simulated using software on the real equipment.
With the same logic, it is possible to create virtual networks 41, . . . , 4K on a given physical network 2, such as that shown in FIG. 1. A link of the physical network 2 may thus carry a plurality of virtual links, a virtual link connecting together two virtual pieces of equipment belonging to the same virtual network.
A virtual network architecture comprises superposing different logical topologies established between virtual pieces of equipment, these virtual topologies being supported by the architecture of the physical network 2.
In the context of increasing offers of service on communications networks, it is becoming less and less possible to envisage allowing all of the data streams using a physical network to travel over identical shortest paths as defined by a protocol such as the interior gateway protocol (IGP). Virtualization thus makes it possible to separate traffic between different types of supported services, by giving them appropriate topologies and quality of service characteristics. As a function of the services it is to support, each of the virtual networks complies with particular constraints, e.g. in terms of quality of service, transit time, or availability. By way of example, such an organization makes it possible to allocate a differentiated service to each of the virtual networks, such as:                a “Voice over IP” service that imposes constraints in terms of end-to-end transit time;        a video on demand service that imposes constraints in terms of data rate and availability;        a service of routing business traffic, that imposes an availability constraint;        a service of routing inter-bank traffic, that imposes security constraints; and        a traffic routing service of the “best effort” type that must not disturb traffic with a guaranteed quality of service.        
In such a model, the topologies of first virtual networks requiring a high quality of service are created on the highest-performance physical paths while those of second virtual networks requiring “best effort” type quality of service are established on physical paths that are little used, a priori. The distribution of the virtual networks as a function of classes of service thus makes it possible to optimize the occupation rates of the physical links.
The data belonging to the various virtual networks is subsequently multiplexed over the physical links. The packets thus pass via the physical interfaces of the physical equipment, where they might potentially give rise to congestion.
Because of this virtual network architecture, a failure of or congestion on a physical link runs the risk of affecting a plurality of virtual links making use of this physical link. The article by Wei Koong Chai et al. entitled: “A policy-driven network management system for the dynamic configuration of military networks” published in the Proceedings of the AIMS 2009 Conference, proposes a method comprising a dynamic allocation of topology per class of service coupled with quality service management between the virtual networks. Implementing such a method involves solving a set of linear equations for minimizing a cost criterion. That gives rise to a large calculation load that should preferably be executed in real time in order to determine metrics associated with the virtual links. That method is also implemented in the event of a physical link failing or becoming congested. Nevertheless, such a method is not suitable for implementing on physical networks of large size, because of the complexity of the calculations.